Heart wall actuation system for the natural heart with shape limiting elements

ABSTRACT

An actuation system for assisting the operation of the natural heart includes an actuator element adapted to be positioned proximate a portion of a heart wall. The actuator element is operable for acting on the heart wall portion to effect a change in the shape of the heart. A shape-limiting element is configured for being positioned proximate a heart wall. The shape-limiting element is operable for flexing to assume a curvature no greater than a predetermined curvature when the heart wall is acted upon and maintaining that predetermined curvature to control the shape of the actuated heart.

RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.10/223,271, filed on Aug. 19, 2002, the entire disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to assisting the natural heart inoperation by actuating a wall of the natural heart, and morespecifically to facilitating such actuation without damage to the hearttissue.

BACKGROUND OF THE INVENTION

The natural human heart and accompanying circulatory system are criticalcomponents of the human body and systematically provide the needednutrients and oxygen for operation of the body. As such, the properoperation of the circulatory system, and particularly, the properoperation of the heart, are critical in the life, health and well beingof a person. A physical ailment or condition which compromises thenormal and healthy operation of the heart can therefore be particularlycritical and may result in a condition which must be medically remedied.

More specifically, the natural heart, or rather the cardiac tissue ofthe heart, can degrade for various reasons to a point where the heartcan no longer provide sufficient circulation of blood for maintainingthe health of a patient at a desirable level. In fact, the heart maydegrade to the point of failure and thereby may not even be able tosustain life. To address the problem of a failing natural heart,solutions are offered to provide ways in which circulation of bloodmight be maintained. Some solutions involve replacing the heart. Othersolutions are directed to maintaining operation of the existing heart.

One such solution has been to replace the existing natural heart in apatient with an artificial heart or a ventricular assist device. Inusing artificial hearts and/or assist devices, a particular problemstems from the fact that the materials used for the interior lining ofthe chambers of an artificial heart are in direct contact with thecirculating blood. Such contact may enhance undesirable clotting of theblood, may cause a build-up of calcium, or may otherwise inhibit theblood's normal function. As a result, thromboembolism and hemolysis mayoccur. Additionally, the lining of an artificial heart or a ventricularassist device can crack, which inhibits performance, even when the crackis at a microscopic level. Moreover, these devices must be powered by apower source, which may be cumbersome and/or external to the body. Suchdrawbacks have limited the use of artificial heart and assist devices toapplications having too brief of a time period to provide a real lastinghealth benefit to the patient.

An alternative procedure also involves replacement of the heart, butincludes a transplant of a natural heart from another human or animalinto the patient. The transplant procedure requires removing an existingorgan (i.e. the natural heart) from the patient for substitution withanother organ (i.e. another natural heart) from another human, orpotentially, from an animal. Before replacing an existing organ withanother, the substitute organ must be “matched” to the recipient, whichcan be, at best, difficult, time consuming, and expensive to accomplish.Furthermore, even if the transplanted organ matches the recipient, arisk exists that the recipient's body will still reject the transplantedorgan and attack it as a foreign object. Moreover, the number ofpotential donor hearts is far less than the number of patients in needof a natural heart transplant. Although use of animal hearts wouldlessen the problem of having fewer donors than recipients, there is anenhanced concern with respect to the rejection of the animal heart.

Rather than replacing the patient's heart, other solutions attempt tocontinue to use the existing heart and associated tissue. In one suchsolution, attempts have been made to wrap skeletal muscle tissue aroundthe natural heart to use as an auxiliary contraction mechanism so thatthe heart may pump. As currently used, skeletal muscle cannot alonetypically provide sufficient and sustained pumping power for maintainingcirculation of blood through the circulatory system of the body. This isespecially true for those patients with severe heart failure.

Another system developed for use with an existing heart for sustainingthe circulatory function and pumping action of the heart, is an externalbypass system, such as a cardiopulmonary (heart-lung) machine.Typically, bypass systems of this type are complex and large, and, assuch, are limited to short term use, such as in an operating room duringsurgery, or when maintaining the circulation of a patient while awaitingreceipt of a transplant heart. The size and complexity effectivelyprohibit use of bypass systems as a long-term solution, as they arerarely portable devices. Furthermore, long-term use of a heart-lungmachine can damage the blood cells and blood borne products, resultingin post surgical complications such as bleeding, thromboembolismfunction, and increased risk of infection.

Still another solution for maintaining the existing natural heart as thepumping device involves enveloping a substantial portion of the naturalheart, such as the entire left and right ventricles, with a pumpingdevice for rhythmic compression. That is, the exterior wall surfaces ofthe heart are contacted and the heart walls are compressed to change thevolume of the heart and thereby pump blood out of the chambers. Althoughsomewhat effective as a short-term treatment, the existing pumpingdevices have not been suitable for long-term use.

Typically, with such compression devices, heart walls are concentricallycompressed. A vacuum pressure is then needed to overcome cardiactissue/wall stiffness, so that the compressed heart chambers can returnto their original volume and refill with blood. This “active filling” ofthe chambers with blood limits the ability of the pumping device torespond to the need for adjustments in the blood volume pumped throughthe natural heart, and can adversely affect the circulation of blood tothe coronary arteries. Furthermore, natural heart valves, between thechambers of the heart and leaching into and out of the heart, are quitesensitive to wall distortion and annular distortion. The compressivemovement patterns that reduce a chamber's volume and distort the heartwalls may not necessarily facilitate valve closure (which can lead tovalve leakage).

Therefore, mechanical pumping of the heart, such as through mechanicalcompression or distortion of the ventricles, must address these issuesand concerns in order to establish the efficacy of long term mechanicalor mechanically assisted pumping. Specifically, the ventricles mustrapidly and passively refill at low physiologic pressures, and the valvefunctions must be physiologically adequate. The myocardial blood flow ofthe heart also must not be impaired by the mechanical device. Stillfurther, the left and right ventricle pressure independence must bemaintained within the heart.

The present invention addresses the issues of heart wall stiffness andthe need for active refilling by assisting in the bending (i.e.,indenting, flattening, twisting, etc.) of the heart walls, rather thanconcentrically compressing the heart walls. Because of the mechanics ofdeformation in hearts having proportions typical in heart failure(specifically, wall thickness/chamber radius ratios), the deformationfrom bending and the subsequent refilling of the heart requiressignificantly less energy than would the re-stretching of a wall thathas been shortened to change the chamber volume a similar amount. Thepresent invention facilitates such desirable heart wall bending andspecifically protects the heart wall during such bending.

Another major obstacle with long term use of such pumping devices is thedeleterious effect of forceful contact of different parts of the livinginternal heart surface (endocardium), one against another, due to lackof precise control of wall actuation. In certain cases, this coaptationof endocardium tissue is probably necessary for a device thatencompasses both ventricles to produce independent output pressures fromthe left and right ventricles. However, it can compromise the integrityof the living endothelium.

Mechanical ventricular wall actuation has shown promise, despite theissues noted above. As such, devices have been invented for mechanicallyassisting the pumping function of the heart, and specifically forexternally actuating a heart wall, such as a ventricular wall, to assistin such pumping functions.

Specifically, U.S. Pat. No. 5,957,977, which is incorporated herein byreference in its entirety, discloses an actuation device for the naturalheart utilizing internal and external support structures. That patentprovides an internal and external framework mounted internally andexternally with respect to the natural heart, and an actuator oractivator mounted to the framework for providing cyclical forces todeform one or more walls of the heart, such as the left ventricularwall. The invention of U.S. patent application Ser. No. 09/850,554,which is also incorporated herein by reference in its entirety, furtheradds to the art of U.S. Pat. No. 5,957,977 and specifically sets forthvarious embodiments of activators or actuator devices which are suitablefor deforming the heart walls and supplementing and/or providing thepumping function for the natural heart.

When heart wall actuation systems like those noted above are utilized,the heart wall is actuated by being indented and/or deformed proximate achamber of the heart to change the volume of the chamber. When actuatedor indented in such a way, a heart wall, or at least portions of thewall may have a tendency to take on shapes that are not desirable from aphysical standpoint. More specifically, the heart walls may have atendency to become overly distended, or take on sharp curvatures, incertain areas based upon the indentation of those walls in other areas.Such unnatural shaping of the heart tissue may be damaging to thetissue. Therefore, when utilizing a heart wall actuation system, oneissue to be addressed is the shape of the walls when the system isactuated, and the variance of that shape from the natural shape that theheart would assume when pumping normally.

It is therefore an objective of the present invention to assist in theoperation of heart wall actuation systems with the natural heart.

It is a further objective to reduce and prevent unnatural distortion ofthe heart and its components during activation with a heart wallactuation system.

It is still another objective of the present invention to providelong-term actuation and assistance for the heart by reducing unnaturalstress on the heart during such actuation.

These objectives and other objectives and advantages of the presentinvention will be set forth and will become more apparent in thedescription of the invention below.

SUMMARY OF THE INVENTION

The present invention addresses the above objectives and otherobjectives by providing an actuation system for assisting the operationof the natural heart which utilizes a shape-limiting element configuredfor being positioned proximate a heart wall to control the shape of theheart when it is actuated. The shape-limiting element is operable forbending or flexing to a predetermined curvature or multiple curvatureswhen the heart wall is acted upon and maintaining those predeterminedcurvatures to control the shape of the actuated heart and limit anyundesirable tensile or compressive strain induced upon the heart tissue.In one embodiment, the shape-limiting element is utilized within anactuation system comprising a framework for interfacing with the naturalheart, which includes an element configured for being anchored to tissueof the heart. An actuator element is adapted for being coupled to theframework and is configured for extending proximate a portion of a heartwall and acting on the heart wall to effect a change in the shape of theheart. The shape-limiting element may be coupled to the actuator elementsuch that the forces on the heart wall are also forces that vary theshape of the shape-limiting element.

In one embodiment of the invention, the shape-limiting element comprisesa plurality of discrete links that are positioned to form an elongatedband. The links are hingedly coupled together and hinge with respect toeach other so the band may flex or bend and change its shape. At leasttwo of the adjacent links are shaped to interfere with each other whenthe links are hinged in a direction for a predetermined distance, tothereby limit further hinging and to maintain a predetermined curvatureof the band. In a more specific embodiment, the adjacent links includeprojections that extend outwardly from a longitudinal axis of the band.The projections are configured for interfering with each other uponflexing or bending of the band, and the resultant hinging of the linksin order to prevent flexing of the band past a certain limit. Theprojections might be configured to further provide one predeterminedcurvature when the band is flexed in one direction, and to provideanother predetermined curvature when the band is flexed in the otherdirection.

In accordance with another embodiment of the present invention, theshape-limiting element is in the form of a flexible belt havingprojections thereon that interfere with each other and limit the flexingof the belt to a predetermined curvature.

In another embodiment of the invention, one or more tethers are utilizedto span between links of a hinging band, or along the flexible belt. Thetethers, which are fixed to the band or belt at certain positions, havelimited extensibility to thereby limit the hinging or flexing of theelement to a predetermined amount or distance to thereby maintain apredetermined curvature of the band when the heart is actuated.

In another embodiment of the invention, a band of discrete hinging linksutilizes a rigid stop element, which spans between the links to engagethe links and limit their hinging to a predetermined amount or distance.Individual links will hinge or pivot until they encounter the rigid stopelement, which generally prevents further hinging. The rigid stopelements might be individual elements, which are coupled between thelinks on one or both surfaces of the band. Alternatively, stop elementson both sides of the band may be coupled together to form a unitarystructure that may be hingedly coupled with the discreet links of theband.

In an alternative embodiment of the invention, a tubular stop element,generally coaxial with the longitudinal axis of the band, may surroundportions of at least two links. The stop elements form upper and lowerstop portions which are figured to engage the links and limit theirhinging to thereby maintain a predetermined curvature of the band, oncethe band is bent or flexed a certain amount or distance.

In another embodiment of the invention, a helical spring is utilized incombination with shape-limiting or curvature limiting structures tolimit the shape of the helical spring and the overall shape of theheart. In one example, a sheath over the helical spring provides suchshape limiting of the helical spring. In another example, discretelinks, interlaced between the coils of the helical spring provide theshape limiting.

Further details of the invention are set forth below in the DetailedDescription of the Invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with a general description of the invention given below, serveto explain the principles of the invention.

FIG. 1 is a perspective view of a human heart utilizing one embodimentof the invention.

FIGS. 2A and 2B are perspective views illustrating an actuation systemincorporating the invention in a relaxed state, and in an actuatedstate, respectively.

FIG. 3 is a sectional view of one embodiment of a shape-limiting elementin accordance with the principles of the present invention.

FIG. 4 is a sectional view of another embodiment of a shape-limitingelement in accordance with the principles of the present invention.

FIG. 5 is a sectional view of another embodiment of a shape-limitingelement in accordance with the principles of the present invention.

FIG. 6 is a sectional view of another embodiment of a shape-limitingelement in accordance with the principles of the present invention.

FIG. 7 is a sectional view of another embodiment of a shape-limitingelement in accordance with the principles of the present invention.

FIG. 8 is a sectional view of another embodiment of a shape-limitingelement in accordance with the principles of the present invention.

FIG. 9 is a sectional view of another embodiment of a shape-limitingelement in accordance with the principles of the present invention.

FIG. 10 is a sectional cross-sectional view of the shape-limitingelement of FIG. 9.

FIG. 11 is a sectional, cross-sectional view of a heart wall actuated byan actuation system without utilizing the shape-limiting invention.

FIG. 12 is a sectional, cross-sectional view of a heart wall utilizingthe present invention.

FIGS. 13A,13B are front views of an alternative embodiment of theinvention shown at rest and bent, respectively.

FIGS. 14A and 14B are front views of another alternative embodiment ofthe invention shown at rest and bent, respectively.

FIG. 15 is a partial perspective view of another embodiment of theinvention.

FIG. 16A, B, C, D, E are partial perspective views of spring elementsused in embodiments of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention may best be described in the context of thenatural human heart, and accordingly, the heart structure is discussedbriefly below. Furthermore, the system of one embodiment is utilizedwith an actuator that is coupled to a framework that cooperates with thehuman heart. One suitable actuator and framework for practicing theinvention is disclosed in greater detail in U.S. Pat. No. 5,957,977,which is incorporated herein by reference in its entirety. Anotheractuation system suitable for use with the present invention is setforth in U.S. patent application Ser. No. 09/850,554, which isincorporated herein by reference in its entirety. A brief overview ofthe heart and a suitable heart wall actuation system for practicing theinvention is set forth below. However, the present invention and itsbenefits are not limited to the disclosed actuation system or framework.Other actuation systems and/or frameworks will also be suitable forpracticing the invention.

Referring now to FIG. 1, a natural human heart 10 is shown inperspective with a portion of a framework for an actuation system. Theheart 10 has a lower portion comprising two chambers, namely a leftventricle 12 and a right ventricle 14, which function primarily tosupply the main pumping forces that propel blood through the circulatorysystem, including the pulmonary system (lungs) and the rest of the body,respectively. Heart 10 also includes an upper portion having twochambers, a left atrium 16 and a right atrium 18, which primarily serveas entryways to the ventricles 12 or 14, and also assist in moving bloodinto the ventricles 12 or 14. The interventricular wall or septum ofcardiac tissue separating the left and right ventricles 12 and 14 isdefined externally by an interventricular groove 20 on the exterior wallof the natural heart 10. The atrioventricular wall of cardiac tissueseparating the lower ventricular region from the upper atrial region isdefined by atrioventricular groove 22 on the exterior wall of thenatural heart 10. The configuration and function of the heart is knownto those skilled in this art.

Generally, the ventricles are in fluid communication with theirrespective atria through an atrioventricular valve in the interiorvolume defined by heart 10. More specifically, the left ventricle 12 isin fluid communication with the left atrium 16 through the mitral valve,while the right ventricle 14 is in fluid communication with the rightatrium 18 through the tricuspid valve. Generally, the ventricles are influid communication with the circulatory system (i.e., the pulmonary andperipheral circulatory system) through semilunar valves. Morespecifically, the left ventricle 12 is in fluid communication with theaorta 26 of the peripheral circulatory system, through the aortic valve,while the right ventricle 14 is in fluid communication with thepulmonary artery 28 of the pulmonary, circulatory system through thepulmonic or pulmonary valve.

The heart basically acts like a pump. The left and right ventricles areseparate, but share a common wall, or septum. The left ventricle hasthicker walls and pumps blood into the systemic circulation of the body.The pumping action of the left ventricle is more forceful than that ofthe right ventricle, and the associated pressure achieved within theleft ventricle is also greater than in the right ventricle. The rightventricle pumps blood into the pulmonary circulation, including thelungs. During operation, the left ventricle fills with blood in theportion of the cardiac cycle referred to as diastole. The left ventriclethen ejects any blood in the part of the cardiac cycle referred to assystole. The volume of the left ventricle is largest during diastole,and smallest during systole. The heart chambers, particularly theventricles, change in volume during pumping.

By way of a non-limiting example, the present invention is discussed interms of embodiments that are used to primarily assist in the actuationand operation solely of the left ventricular portion of the heart 10.However, it is noted that the present invention can also be used toassist in the actuation and operation of other portions of the naturalheart 10, such as individual atria, the right ventricular portion of theheart 10, or simultaneously both atria or both ventricles.

In accordance with illustrating an example of use of the invention withthe left ventricular portion of the heart, one possible framework andactuator system are discussed which are positioned on the exteriorsurface or epicardium of the left ventricle. The invention may also beused with other chambers of the heart.

Part of the framework for an actuation system is illustrated in FIGS. 1,2A, and 2B by reference numeral 70, which refer to an external componentor yoke of the framework. The framework also includes one or moreinternal framework elements (not shown) including an internal stent towhich the external yoke or external framework element 70 is fixed bytransmural cords, which extend through walls of the heart. The internalstent is sized and configured for placement within the interior volumeof the natural heart 10, generally alongside the right side of theinterventricular septum. The stent also includes at least two separatering structures for positioning proximate the valve annuli of the leftside of the heart. Further details of one suitable framework are setforth in U.S. Pat. No. 5,957,977.

As noted above, the framework includes external yoke 70, for placementaround a portion of the exterior surface or epicardium of a naturalheart 10. The generally stirrup-shaped yoke 70 in the illustratedembodiment restricts free motion of the natural heart 10 so that thenatural heart 10 can be actuated and assisted. Yoke 70 also acts as ananchor or base for an appropriate actuator system for use with theinvention. In one embodiment, the yoke 70 is between about 1 and 2 cmwide and includes a semi-rigid collar portion, preferably made of eithera solid polymer of appropriate mechanical behavior, such aspolypropylene or polyacetal, or a composite of metal (stainless steel orpure titanium) band or coil spring elements, polymer fabric and fiber(e.g. polyester knit) and soft elastomer, for providing rigidity to theyoke 70. Additionally, the yoke 70 may include a gel-filled cushionportion 80 that is positioned immediately adjacent the exterior surface(epicardium) of the natural heart 10 for providing equalized pressureover the irregularities in the epicardial surface of the heart 10, andany of the coronary arteries 30 within a region under the yoke 70.Preferably, the yoke 70 is sized and configured for placement adjacentat least a portion of the atrioventricular groove 22, and simultaneouslyadjacent at least a portion of the anterior and posterior portions ofthe interventricular groove 20, and most preferably, adjacent at least asubstantial portion of the anterior and posterior portion of theinterventricular groove 20, as shown in FIG. 1.

General alignment of the yoke 70 with interior framework elements ismaintained by at least one transmural cord (not shown), and preferably,a plurality of cords that penetrate the walls of the natural heart 10and connect to the internal stent and one or more of the internal rings,as discussed in U.S. Pat. No. 5,957,977.

FIGS. 2A and 2B illustrate one embodiment of the present invention whichincludes elements of the framework described above, and specificallyincludes the external framework element or yoke 70, anchored to tissueof the heart. The actuation system 72 includes an actuator element,which is configured to engage or extend proximate to or along a heartwall exterior surface, or epicardial surface 73 of the heart 10 (SeeFIG. 1). The actuation system 72 has a relaxed state as illustrated inFIG. 2A, wherein the actuator element, such as an actuator band 74 willgenerally follow the distended curvature of a heart wall portion of therelaxed or diastolic heart. The actuation system also has an actuatedstate, as illustrated in FIG. 2B, wherein the band 74 engages and actson the outer surface of the heart wall and effects a shape and volumechange of a portion of the heart, such as the left ventricle byindenting or deforming the heart wall. The shape of the actuated band 74will determine the type of shape of the heart indentation and theresultant forces on the heart. In the embodiment of the inventionillustrated in FIGS. 2A and 2B, the actuation system thereby comprisesan actuator band 74 which is selectively movable between the relaxedstate (FIG. 2A) and actuated state (FIG. 2B). The actuator band 74 isoperable, when in the actuated state, to assume a predetermined shapeand/or curvature, and thereby indent a portion of the heart wall toeffect a change in the shape of the heart and thereby effect a reductionin the volume of the heart chamber. A drive apparatus 76 is coupled tothe actuator band and is operable for selectively moving the actuatorband between the relaxed and actuated states to achieve the desiredassistance of the natural heart.

Referring to FIG. 2A, the actuation system utilizes an actuator band 74comprising a plurality of juxtaposed blocks 78 which may be drawntogether by drawing cords 80, which pass through apertures 81 in theblocks, and through associated sheaths 82, to make the band 74 form apredetermined shape and thereby act on the heart surface 73 to effect achange in the shape of the heart.

In accordance with one aspect of the present invention, the shape of theheart is controlled by shape-limiting elements 86 which are alsoconfigured for being positioned proximate a heart wall. Theshape-limiting elements will generally be positioned proximate theactuator element. In the embodiment illustrated in FIGS. 2A and 2B, theshape-limiting elements 86 cooperate with the actuator element or band74 and are actually coupled thereto. The shape-limiting elements areoperable for bending or flexing to assume a predetermined curvature orshape when the heart wall is acted upon by band 74. The shape-limitingelements 86 maintain the predetermined curvature or shape and controlthe shape of the heart so that the heart is not overly distended, and sothat the heart will not take on shapes or curves, which are detrimentalto the heart wall, and the tissue forming the heart wall.

In one embodiment of the invention, the shape-limiting elements areconfigured as elongated bands, which extend along a wall of the heart.The bands are flexible and may be bent or flexed when a force is appliedthereto, such as by an actuator element 74. However, the bands areconstructed to only bend or flex a certain amount or a certain distance,and then to resist any further bending or flexing. That is, variousportions of the flexible band will take on or assume predeterminedcurvatures when bent or flexed. Once the predetermined curvature isattained, the curvature-limiting band will resist any further bending orflexing in that particular direction. In accordance with one aspect ofthe present invention, different portions or sections of the band mayhave different predetermined curvatures or shapes when bent. That is,one portion might bend past the curvature amount of another portion ofthe band. Furthermore, the band might flex in one direction a greateramount than it flexes in another direction, so that, depending upon thedirection of flex, the band will take one shape, or have onepredetermined curvature which is different than the shape orpredetermined curvature achieved when the band is flexed in the otherdirection.

As noted above, the shape-limiting elements, as illustrated herein, arenot specifically confined to use with a “string-of-blocks” actuatorelement 74, which is shown by way of example. As illustrated in FIGS. 2Aand 2B, the shape-limiting elements 86 are coupled to the actuatorelement 74 for moving with the actuator element and being bent or flexedby the forces of the actuator element. Alternatively, they might bemounted and positioned proximate the heart independently of the actuatorelement.

FIG. 3 illustrates one embodiment of a shape-limiting element inaccordance with the principles of the present invention. Figure 3 showsa perspective view of a small section of an elongated band 88, whichcomprises a plurality of distinct links that are positioned, generallyend-to-end or side-by-side to form an elongated band. The links arehingedly coupled together to hinge with respect to each other forvarying the shape of the band. For example, the links 90 a, 90 b, inFIG. 3 have tabs 91 with apertures 92 formed therethrough for receivinghinge pins 93 such that the links are hingedly coupled together to hingewith respect to each other for varying the shape of the band. The linksinclude projections or projection sections 94, 96 that extend outwardlyfrom a longitudinal axis of the band indicated by reference numeral 97.When the adjacent links, such as links 90 a and 90 c are hinged acertain distance in a direction, such as the direction indicated byreference arrows 98, the links interfere with each other to limitfurther hinging and to maintain a predetermined shape and/or curvatureof the band. That is, when the links are hinged in the direction 98, theprojections 94 will touch each other and prevent further hinging, tolimit the curvature of the bands. In that way, the actuated or flexedband will maintain a predetermined curvature, and when positioned alongthe side of the heart, the band 88 will control the shape of the heartwhen the heart wall is actuated by the actuator mechanism.

As illustrated in FIG. 3, the links 90 a, 90 b, and 90 c haveprojections 94, 96 that extend on both sides of a longitudinal axis 97.Therefore, when the adjacent links 90 a, 90 c are hinged in an oppositedirection as indicated by reference numeral 99, projections 96 willinterfere with each other. When the adjacent links are hinged apredetermined amount or distance, further hinging is thereby limited tomaintain a predetermined curvature in the band in that direction aswell. Therefore, the adjacent links are shaped to interfere with eachother when hinged in both the first direction and also in an oppositedirection. Alternatively, the links might be configured only tosignificantly interfere with each other when the band is flexed in onedirection. As will be appreciated, each of the adjacent links willgenerally interfere with the other adjacent links to limit hinging pasta certain point and thereby maintain the predetermined curvature of theband. That is, band 88 will flex and bend for predetermined distances incertain sections of the band to form a shape and to maintainpredetermined curvatures. The band will then no longer significantlyflex or bend past a predetermined curvature, and the predeterminedcurvature of the band will be maintained to maintain a desired shape ofthe heart.

In accordance with another aspect of the present invention, the adjacentlinks may be further configured to maintain one predetermined curvaturewhen hinged in one direction, and to maintain a different predeterminedcurvature when hinged in an opposite direction. Referring again to FIG.3, the projections 96 illustrated for the links are thinner than theprojections 94. In that way, the hinged links, specifically projections96, will have to hinge a greater amount in one direction before theyinterfere with each other, than will the projections 94 (which aregenerally illustrated as being wider than the protrusions 96) when theband is hinged in an opposite direction. As such, differentpredetermined curvatures are allowed, depending upon which direction theactuator band is flexed or bent.

Turning now to FIGS. 11 and 12, a schematic view is shown of heart wallsurface 73 being actuated by an actuator element 74 with and without theshape-limiting of the present invention. Specifically, FIG. 11 showssignificant curves in the heart wall surface 73 made by the actuatorelement 74 when the shape of the heart is not controlled. FIG. 12, onthe other hand, shows a system utilizing a shape-limiting element 100,which maintains a predetermined curvature and controls the shape of theactuated heart. To that end, the curvature (mathematically defined asthe inverse of the radius of curvature) associated with the curves 101,as illustrated in FIG. 11, is significantly lessened for the curve 102,as illustrated in FIG. 12. In that way, significantly unnatural shapingof the heart is prevented, and damage to heart tissue is also prevented.

FIG. 5 illustrates an alternative embodiment of the shape-limitingelement of the invention. The element is in the form of a band 104,including a flexible belt 106, with a plurality of protections, orprojection portions 108, which extend outwardly from a longitudinal axis107 of the band. As discussed above with respect to FIG. 3, the band 106is flexible, and can flex or bend in opposite directions for apredetermined amount to form a predetermined shape and curvature, suchas when the heart wall is acted upon by an actuator element. Theprojections 108, which are shown on both sides of the belt 106, in theembodiment of FIG. 5, interfere with each other when the belt is flexeda predetermined amount to limit further flexing and to maintain apredetermined curvature of the band. In that way, the shape of the heartmay be controlled when a wall of the heart is being actuated by anactuation system. The belt 106 may be formed of a suitable flexiblematerial, such as a strip of soft, solid polymer, leather, fabric, andso forth. The projections 108 may be formed of a suitable sufficientlyhard material, such as stainless steel 316, or CP titanium, so that theprojections properly interfere with each other to prevent furtherflexing or hinging of the band as discussed above with band 88.Similarly, the links 90 of band 88 may be formed of such materials.

In accordance with another aspect of the present invention, thepredetermined curvature of the shape-limiting element may be maintainedby tether structures spanning between at least two adjacent links,rather than mechanical interference between the links. Turning to FIG.4, a band 110 is illustrated, having multiple discreet links 112 whichmay hinge with respect to each other on pins 113. A tether 114 spansbetween at least two adjacent links. The tether 114 has limitedextensibility to limit hinging when the adjacent links 112 are hinged ina direction a predetermined amount or distance. To that end, the tether114 may be fixed at points 116 to a side of the band 110. Alternatively,another, similar tether might be positioned on the other side of theband 110. As is readily understood, when the links 112 are hinged in onedirection to extend a portion of the tether 114, such as tether portion114 a, that portion will reach its maximum extended length and thenlimit further hinging of the links in a particular direction. It is thetether on the outside radius of the curved band which limits furtheraction, rather than the tether on the inside radius. By tetheringvarious of the links of the band 110 together, the predeterminedcurvature of the band may be maintained. FIG. 4 illustrates a singletether 114 extending along the length and coupled at multiple points116. Alternatively, individual small tether portions 114 a might beindividually attached between the links 112, rather than a single longtether. With tethering on both sides, the predetermined curvature of theband may be controlled when the band is bent or flexed in bothdirections.

FIG. 6 illustrates another alternative embodiment of a shape-limitingelement, wherein a band 118 includes a belt 120 and a tether 122, whichis coupled at various points 123 along the length of the belt. When thebelt is flexed or bent in a direction, tether 122 has limitedextensibility and will only allow the belt 120 to bend to apredetermined curvature, and will then maintain that predeterminedcurvature and generally prevent further bending in order to control theshape of the heart that is being actuated.

The tethers 114, 122, as illustrated in the drawings, are in the form ofthin bands. However, the tethers might include alternative structures,such as cord, cables and chains. Multiple tethers or a single tether arefixed to the surfaces of a band or belt, and fixed in intervals to suchsurfaces. At the extent of flexion of the band or belt between tetherfixation points. The corresponding segment of tether becomes taut andthe belt or band flexion is limited to maintain a predeterminedcurvature.

Turning now to FIG. 7, a band 124, having multiple links 126 is shownsimilar to the band of FIG. 4. The multiple links or other discreteelements are positioned to form an elongated band, and the links hingewith respect to each other for varying the shape of the elongated band.For maintaining a predetermined curvature, band 124 utilizes a pluralityof rigid stop elements 128 which are fixed to the band to span betweenat least two adjacent links. The stop elements 128 are configured toengage the links and limit their hinging when the adjacent links arehinged in one direction a predetermined amount. This thereby maintains apredetermined curvature of the band. In the embodiment illustrated inFIG. 7, a single stop element 128 fixed to one link 126 also spansacross and engages adjacent links on either side of the link to whichthe stop element is fixed. The stop element may be shaped and configuredto provide a desired amount of flexing or bending of the band, toprevent any further flexing or bending past the predetermined curvature,as discussed above. The stop elements 128 might be positioned on one orboth surfaces of the band 124.

FIG. 8 illustrates another alternative embodiment of a shape-limitingelement wherein band 130 includes discrete links 132, which hinge withrespect to one another, such as at a hinge point 133. Band 130 betweenthe links 132 incorporates a structure 134 which incorporates stopelements 136 on both sides of the band, and which couples the stopelements 136 together. The structure 134 has an I-beam shape incross-section. The center portion 137 between the stop elements 136 maybe coupled to the links 132, such as by being hingedly coupled with thelinks, or may float freely between the links. In any case, when thelinks are hinged together and the band is bent or flexed, the links 132will engage the stop structures 136 and be prevented from furtherhinging past a predetermined curvature for the band, in accordance withone aspect of the present invention.

FIGS. 9 and 10 illustrate another embodiment of a stop structure, whichmight be utilized with a shape-limiting element, for achieving thedesired predetermined curvature and flexion limitation for theshape-limiting element.

FIGS. 9 and 10 illustrate a perspective and cross-sectional view,respectively, of a band 140 comprising a plurality of links 142, such asthe link structures as illustrated in FIGS. 4 and 7. Rather than havingstop structures individually positioned proximate sides of the band, atubular-shaped stop element 144 is utilized, which forms an opposing topstop portion 146 a and a bottom stop portion 146 b. The tubular-shapedelement 144 cooperates to prevent hinging of the links 142 beyond apredetermined amount to thereby set and maintain a predeterminedcurvature for the band when it is flexed or bent in one or moredirections. The tubular stop element 144 might be fixed in place, suchas by being hinged with the center linkage element 143 at a hinge point147, as illustrated in FIGS. 9 and 10. Alternatively, rather than havinga specific linkage element, the links 142 may be directly hinged to eachother on a hinge pin that is mounted to the tubular stop element 144 athinge point 147. Referring to FIG. 10, when the links 142 hinge past acertain point, various of the links will engage the tubular stop element144 and generally be prevented from hinging further in a particulardirection.

In accordance with one advantage of the present invention, the imposedcurvature of a heart wall, which is deformed by a heart wall actuationsystem and an actuator element, cannot exceed a given limit. Thecurvature “k” is defined as the inverse of the radius of curvature withunits of length. By preventing the curvature (k) from exceeding a givenlimit, the radius of curvature is prevented from being reduced below agiven limit, so that sharp curvature points of the deformed or actuatedheart wall are avoided.

This is a particular advantage, because the maximum tensile orcompressive strain induced in the heart wall tissue is a directconsequence of the thickness of the heart wall and the inducedcurvature. Any excessive tensile or compressive strain on the heart wallmay cause tissue disruption or other associated damage.

Referring to FIGS. 11 and 12, various crossing lines 103 are illustratedthrough the heart wall 73. The change and separation between the variouscrossing lines 103, at varying positions along the heart wall, indicatethe imposed tensile and/or compressive strain at those points. As may beseen between FIGS. 11 and 12, the maximum tensile strain in the innerlayers of portions of the heart wall 73 adjacent the corners of theactuator element 74, are much greater without a shape-limiting element100 (FIG. 11), than with a shape-limiting element (FIG. 12).

In accordance with another aspect of the present invention, flexiblespring structures, combined with additional structures to limit theirflexing or bending, may be utilized. Such structures may be used asshape-limiting elements as discussed above. Referring to FIGS. 13A and13B, a flexible spring 160 is shown and includes closely wound ortightly wound coils or coil turns. Spring 160 may be made of highfatigue metal, such as titanium, or other suitable compositions. Also,the spring might be constructed with standard circular turns or coils asillustrated in FIGS. 13A and 13B. Alternatively, as illustrated in FIG.16A-E and as illustrated and further discussed here and below, othercoil shapes might be used.

In combination with spring structure 160 is an outer sheath 162, whichis woven or otherwise formed around the spring structure 160. The sheath162 operates to restrict the bending or flexibility of the inner corespring structure 160. The spring structure 160 may be bent until thesheath 162 reaches its maximum elongation. The sheath is specificallywoven or formed to allow bending of the spring structures 160 within arange designated by the curvature desired. Therefore, curvature of thespring structure is limited.

More specifically, referring to FIG. 13B, when the spring structure isbent, a portion 164 thereof becomes more convex (or less concave) andthe distance between the individual coil sections 165 increases. Thedistance between the coil sections 165, and therefore the shape of thespring structure 160 at area 164, is limited by the restriction providedby sheath 162. Similarly, a portion or area 166 of the spring structure160, indicated by coil sections 167, becomes generally more concave (orless convex) wherein the spring structure in that area remains generallythe same length as it was prior to being bent as illustrated in FIG.13A. The outer sheath 162 provides generally a tethering action and whenany part of the sheath reaches its maximum length (e.g., all componentfibers within a woven sheet are straightened to their maximum length),further bending or flexing is generally prevented. Generally, the sheathis constructed so that substantial deformation or breaking of its fibersor other components will not occur with the forces that are expected inits use. Referring to FIG. 1, the structure such as illustrated in FIGS.13A and 13B might be utilized as a shape-limiting element 86 as shown inFIG. 1.

FIGS. 14A and 14B illustrate another alternative embodiment of a springstructure 170, in combination with a sheath 172. The spring structure170 has loosely wound coils 174. It is also formed of a suitablematerial, such as CP titanium or stainless steel or a shape memorycomposition. The spring structure 170 is encased, at least partially, ina sheath 172 which may be similarly formed as the sheath 162 discussedabove. When a bending moment is imposed on the spring structure 170, aconvex surface 176 of the sheath 172 is already tightened and will notpermit the convex coil sections 178 to generally separate any further.This generally prevents significant elongation of the convex portion ofthe spring. However, in a concave portion 179 of the spring structureillustrated by coil sections 180, the coil sections 180 may be drawncloser together, thereby effectively shortening concave portion 179.Generally when the coiled sections 180 are in contact with each other,no further significant shortening of the concave portion 179 will occurwithout either compressive deformation of the coil sections 180 ortensile deformation or breaking of the sheath 176. The structure may bedesigned and build for specific minimum radius (or, equivalently,maximum curvature) limits by selection of wire gauge, inter-coil coilgaps after sheath placement, and elastic characteristics of the sheath,and initial sheath tension imposed by the compressed spring structure.

In an alternative embodiment of the invention as illustrated in FIG. 15,a somewhat loosely wound helical spring structure 184 might beconstrained along a side or portion thereof to maintain a specificlength or curvature. Referring to FIG. 15, the coil structure 184 havingvarious coil turns 186 is utilized in combination with rigid links 188which connect adjacent coils along one side, or section, or aspect ofthe coil structure 184. The links are formed of a suitable rigidmaterial, such as a high-strength polymer (e.g., polyacetal or ultrahighmolecular weight polyethylene (UHMWPH)), or of a highly rigid materialsuch as a metal (e.g., stainless steel or CP titanium or otherbiocompatible alloy). The links 188 are formed so that they haveappropriate apertures 190 through which the coil turns 186 may pass. Theapertures 190 may be angled to address the appropriately angled pitch ofthe coil turns. As illustrated in FIG. 15, adjacent coils, such as coils186 a and 186 b are kept at a fixed distance apart, along one aspect ofthe spring structure, by one or more of the links 188. In oneembodiment, the links will be staggered.

For example, links 188 a and 188 b might couple together turns 186 a and186 b. Similarly, link 188 c might couple together turns 186 a and 186c. Further, links 188 d and 188 e might couple together coil turns 186 cand 186 d. Similarly, the pattern might be repeated along the desiredlength of the spring structure 184. In that way, the linked aspect orside of the spring structure has a fixed length, but can change itscurvature. If the bending moment caused the spring structure 184 tocurve, there would generally be little resistance other than therelatively low flexural rigidity of the coil spring, until the coilportions on the aspect opposite the linked aspect came in contact witheach other. Then a compressive force would prevent further substantialbending of the spring structure unless either the links or theirrespective wire portions fail in tension or shear, the lower-aspect wiresegments fail in compression, or the entire spring buckles. Inaccordance with one aspect of the present invention, the design andmaterial choice would be made such that any failure modes would behighly unlikely under expected loading or curvature. The links might bekept in position by a retaining feature, which is secured appropriatelyto the link structures on the side of the coil.

Alternatively, they might be allowed to move freely on the coilstructure.

In the still further alternative embodiment, a fabric sheath, such asthat described above with respect to FIGS. 13A, 13B, 14A, and 14B mightbe utilized with the structure illustrated in FIG. 15. The sheath incombination with the links 188 might be utilized to further limitcurvature of the structure illustrated in FIG. 15.

FIGS. 16A-16E illustrate various embodiments of a spring structure,which might be utilized as described herein as a curvature-limitingelement. That is, these figures illustrate various different turn orcoil shapes which might be utilized rather than simply circular coilshapes and turns of a standard helical coil. For example, the coils ofspring structure 200, FIG. 16A, are generally triangular, while thecoils of spring structure 202 are generally trapezoidal. Springstructure 204 has round coils and spring structures 206, 208 have coilsthat are rectangular with rounded corners and rectangular with roundedends, respectively. It will be understood by a person of ordinary skillin the art that other spring structures might also be utilized.

While the present invention has been illustrated by the description ofthe embodiments thereof, and while the embodiments have been describedin considerable detail, it is not the intention of the applicant torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details representative apparatusand method, and illustrative examples shown and described. Accordingly,departures may be made from such details without departure from thespirit or scope of applicant's general inventive concept.

1. An actuation system for assisting the operation of the natural heart,the actuation system comprising: an actuator element configured forbeing positioned proximate a portion of a heart wall and operable foracting on the heart wall portion to effect a change in the shape of theheart; a shape-limiting element configured for being positionedproximate a heart wall, the shape-limiting element operable for flexingto assume a predetermined curvature when the heart wall is acted uponand maintaining that predetermined curvature to control the shape of theheart.
 2. The actuation system of claim 1 wherein shape-limiting elementis coupled to the actuator element.
 3. The actuation system of claim 1wherein the shape-limiting element comprises: a plurality of discretelinks positioned to form a band, the links hingedly coupled together tohinge with respect to each other for varying the shape of the band; atleast two adjacent links being shaped to interfere with each other, whenthe adjacent links are hinged in a direction for a predetermineddistance, to limit hinging and to maintain a predetermined curvature ofthe band.
 4. The actuation system of claim 3 wherein the adjacent linksinclude projections extending outwardly from a longitudinal axis of theband, the projections configured for interfering with each other.
 5. Theactuation system of claim 3 wherein said adjacent links are shaped tointerfere with each other when hinged in both one direction and anotherdirection.
 6. The actuation system of claim 5 wherein the adjacent linksare further configured to maintain one predetermined curvature whenhinged in the one direction and to maintain a different predeterminedcurvature when hinged in the another direction.
 7. The actuation systemof claim 1 wherein the shape-limiting element comprises: a plurality ofdiscrete links positioned to form a band, the links hingedly coupledtogether to hinge with respect to each other for varying the shape ofthe band; a tether spanning between at least two links, the tetherhaving limited extensibility to limit hinging when the links are hingedin a direction for a predetermined distance to thereby maintain apredetermined curvature of the band.
 8. The actuation system of claim 1wherein the shape-limiting element comprises: an elongated, flexiblebelt; a plurality of projections extending outwardly from a longitudinalaxis of the belt, the protrusions configured for interfering with eachother, when the belt is flexed in a direction for a predetermineddistance, to limit flexing and to maintain a predetermined curvature ofthe belt.
 9. The actuation system of claim 1 wherein the shape-limitingelement comprises: an elongated, flexible belt, a tether fixed to thebelt in at least two positions spaced along a longitudinal axis of thebelt, the tether having limited extensibility to limit flexing of thebelt when the belt is flexed in a direction for a predetermined distanceto thereby maintain a predetermined curvature of the belt.
 10. Theactuation system of claim 1 wherein the shape-limiting elementcomprises: a plurality of discrete links positioned to form a band, thelinks hingedly coupled together to hinge with respect to each other forvarying the shape of the band; a rigid stop element spanning between atleast two links, the stop element configured to engage the links andlimit hinging, when the links are hinged in a direction for apredetermined distance, to thereby maintain a predetermined curvature ofthe band.
 11. The actuation system of claim 10 further comprising arigid stop element spanning the links on opposite sides thereof andlimiting hinging in both one direction and another direction.
 12. Theactuation system of claim 11 wherein the stop elements on the oppositesides of the links are further configured to maintain one predeterminedcurvature when the links are hinged in the one direction and to maintaina different predetermined curvature when the links are hinged in theanother direction.
 13. The actuation system of claim 11 wherein the stopelements on the opposite sides are coupled together between the links.14. The actuation system of claim 13 wherein the stop elements arecoupled together to form a generally unitary structure.
 15. Theactuation system of claim 1 wherein the shape-limiting elementcomprises: a plurality of discrete links positioned to form a band, thelinks hingedly coupled together to hinge with respect to each other forvarying the shape of the band; a tubular stop element surroundingportions of at least two links and generally coaxial with a longitudinalaxis of the band, the stop element configured to engage the links andlimit hinging, when the adjacent links are hinged in a direction for apredetermined distance, to thereby maintain a predetermined curvature ofthe band.
 16. A method for assisting the operation of the natural heart,the method comprising: positioning an actuator element proximate to aheart and operating the actuator element to act on a heart wall portionto effect a change in the shape of the heart; with a shape-limitingelement configured for being positioned proximate a heart wall, allowingthe shape-limiting element to flex to a predetermined curvature when theheart wall portion is acted upon and maintaining that predeterminedcurvature to control the shape of the actuated heart.
 17. The method ofclaim 16 further comprising flexing the shape-limiting element with theoperation of the actuator element.
 18. The method of claim 16 whereinthe shape-limiting element comprises a plurality of discrete linkspositioned to form a band, the links hingedly coupled together to hingewith respect to each other for varying the shape of the band, the methodfurther comprising limiting the hinging of links of the band to maintaina predetermined curvature of the band.
 19. The method of claim 18further comprising limiting the hinging of the links with projections onat least two links wherein the projections interfere with each otherwhen the links are hinged.
 20. The method of claim 18 further comprisinglimiting the hinging of the links with at least one tether spanningbetween at least two links and having limited extensibility.
 21. Themethod of claim 16 wherein the shape-limiting element comprises anelongated, flexible belt, the method further comprising limiting theflexing of the belt to maintain a predetermined curvature of the belt.22. The method of claim 21 further comprising limiting the flexing ofthe belt with a at least one tether fixed to the belt in at least twospaced positions along the belt, the tether having limitedextensibility.
 23. The method of claim 18 further comprising limitingthe hinging of the links with at least one rigid stop element spanningbetween at least two links, the stop element configured to engage thelinks and limit hinging.
 24. The method of claim 18 further comprisinglimiting the hinging of the links with at least one tubular stop elementsurrounding portions of at least two links, the stop element configuredto engage the links and limit hinging.
 25. The method of claim 16wherein the shape-limiting element comprises a spring structureincluding a plurality of turns and at least one link configured tocouple with turns of the spring structure to limit the separation of theturns when the spring structure is flexed.